Devices for providing a liquid additive are used, for example, in the field of motor vehicles in order to feed a liquid additive to an exhaust gas treatment device for purifying the exhaust gases of an internal combustion engine of the motor vehicle. Exhaust gas treatment devices in which a liquid additive is used for the purification of exhaust gases are widespread. An exhaust gas purification method which is carried out particularly frequently in such exhaust gas treatment devices is the method of selective catalytic reduction (SCR method). In this method, nitrogen oxide compounds in the exhaust gas are reduced using a reducing agent. In this context, ammonia is typically used as the reducing agent. The exhaust gas treatment device typically has an SCR catalytic converter at which the nitrogen oxide compounds in the exhaust gas are reduced using the ammonia. Ammonia is generally not stored directly in motor vehicles but instead in the form of a reducing agent precursor solution. This reducing agent precursor solution is a liquid additive. A particularly frequently used reducing agent precursor solution is a urea/water solution. A 32.5% urea/water solution is available with the tradename AdBlue®.
In the case of an operational start of such a device, it is problematic that this liquid additive can freeze at low temperatures. The urea/water solution described above freezes, for example, at −11° C. Such low temperatures can occur, in particular, during a long phase of deactivation of the motor vehicle. After a long phase of deactivation the liquid additive in the device may have completely frozen. The device is then initially incapable of providing any liquid additive. It is known that devices for providing liquid additive have a heating system for melting frozen liquid additive, so that liquid additive can be provided promptly after the operational start.
In particular PTC heating elements (PTC=Positive Temperature Coefficient) are proposed as the means of heating such devices. PTC heating elements are electric heating elements which are heated by a through-flowing electric current. They have the additional property that the electrical resistance of the current increases as the temperature rises. It is therefore ensured that the electric current is automatically reduced at high temperatures. As a result of the reduction in the electric current, the heating capacity is also reduced. This constitutes an automatic protection of a PTC heating element against overheating.
In these devices with PTC heating elements, insufficient conduction away of the heat from the PTC heating element is problematic, since the PTC heating element heats up to a high temperature within a short time and subsequently only conducts a small electric current. The heating capacity is therefore limited in a self-regulating fashion within a short time, with the result that the desired heating function is satisfied only to a small degree.